In recent years, a variable valve actuation control (VVC) system, capable of variably adjusting a valve lift and valve timing of at least one of intake and exhaust valves of an internal combustion engine depending on engine operating conditions, is widely utilized for controlling a charging efficiency, an effective compression ratio, and an amount of residual gas of the engine, thereby enhancing the combustion performance and engine power performance and exhaust emission control performance. In gasoline engines, the premixed air-fuel mixture is ignited by means of a spark plug. Recently, there have been developed and advanced studies of “compression ignition engine technologies”. In Diesel engines or premix compression ignition engines including gasoline engines, air alone is compressed during the compression stroke, and then fuel, which is sprayed or injected into the cylinder, is self-ignited due to a temperature rise of the compressed air (heat produced by compressing the incoming air).
The ignition lag, which controls or manages the state of combustion, changes depending on the temperature and pressure of air-fuel mixture, and various states or various characteristics of air-fuel mixture, for example, a turbulence intensity, and a fuel property. In other words, a deviation of the ignition lag from an optimal value varies depending on the air-fuel mixture temperature, air-fuel mixture pressure, turbulence intensity in the combustion chamber, fuel property, and the like. By means of the VVC system, it is possible to adjust-the-effective compression ratio, thereby compensating for a deviation of the ignition lag from an optimal value. As a consequence, it is possible to optimally control the state of combustion by means of the VVC system. Note that the effective compression ratio is correlated to a geometrical compression ratio but differs from the geometrical compression ratio. The geometrical compression ratio often denoted by Greek letter “ε” is generally defined as a ratio (V1+V2)/V1 of the full volume (V1+V2) existing within the engine cylinder and combustion chamber with the piston at a bottom dead center (BDC) position to the clearance-space volume (V1) with the piston at a top dead center (TDC) position. On the other hand, the effective compression ratio denoted by Greek letter “ε′” is generally defined as a ratio of the effective cylinder volume corresponding to the maximum working medium volume to the effective clearance volume corresponding to the minimum working medium volume. These two compression ratios ε and ε′ are thermodynamically distinguished from each other. The state of combustion is affected by various factors as well as effective compression ratio ε′, for example, the air-fuel mixture temperature, and thermodynamic and hydrodynamic properties or characteristics (boost pressure created by a super-charging system, super-charged air temperature, cooling characteristics in a cooling system, an amount of deposits adhered to the cylinder wall, an amount of external EGR (exhaust gas recirculated), and the like). Additionally, for the same intake valve closure timing, the effective compression ratio ε′ is affected by several factors, for example the air-fuel mixture temperature at the beginning of compression stroke, the air-fuel mixture pressure at the beginning of compression stroke, and the EGR amount.
There have been proposed and developed various multistage fuel-injection control technologies for Diesel engines or in-cylinder direct-injection gasoline engines in which fuel is injected directly into the engine cylinder. According to such multistage fuel-injection control, fuel injection of one operating cycle of events is split to several times (a plurality of fuel-injection pulses), for the purpose of good air/fuel mixture blending and improved combustion. A fuel injection pattern of one cycle is classified into a plurality of injection areas, for example, a pilot-injection area, a main-injection area, and an after-injection area. Japanese document “JSAE Journal Vol. 58, No. 4, 2004, pp 19-24” published by Society of Automotive Engineers of Japan, Inc. and titled “1800 bar Common Rail System for Diesel Engine” and written by two authors Yasushi Tanaka and Koji Nagata, teaches a fuel injection system employing a high-response high-speed high-precision fuel-injection valve capable of injecting a very small amount of fuel at a very short injection interval. In the case of the high-response fuel-injection valve described in the Japanese document “JSAE Journal Vol. 58, No. 4, 2004, pp 19-24”, during the combustion stroke for one engine cylinder, fuel injection of five times at the maximum can be executed. By way of high-precision fuel injection of multiple times for each engine operating cycle, it is possible to realize the reduced exhaust emissions and reduced combustion noise. For instance, when subsidiarily injecting a very small amount of fuel before main injection, it is possible to simultaneously reduce nitrogen oxides (NOx) and particulate matter (PM) contained in the exhaust gases. Furthermore, subsidiary injection of a very small amount of fuel (hereinafter is referred to as “sub-injection”) taking place after main injection, results in a rise in exhaust gas temperature, that is, a catalyst temperature rise. This contributes to (i) rapid catalyst activation during cold engine operation, and (ii) burning of the particulate matter (PM) accumulated in a Diesel particulate filter, often abbreviated to “DPF”, that is, removal of the PM emissions from the DPF, in other words, DPF regeneration.
The timing of sub-injection is determined based on engine operating conditions and electronically controlled over wide range of conditions under which the engine operates. In particular, in premix compression ignition engines, the ignition of air-fuel mixture is controlled by way of the intake valve closure timing, often denoted by “IVC” and expressed in terms of crank angle. In more detail, it is possible to variably adjust the mass of air entering the engine cylinder at the beginning of compression stroke by retarding or advancing the intake valve closure timing. It is possible to retard a rise in in-cylinder pressure and a rise in in-cylinder temperature with respect to a predetermined crankangle. In other words, it is possible to lower the effective compression ratio ε′ by retarding an in-cylinder pressure rise and/or an in-cylinder temperature rise by way of variable adjustment of intake valve closure timing IVC. One such IVC adjustment type variable compression ratio device for a compression ignition engine has been disclosed in Japanese Patent Provisional Publication No. 1-315631 (hereinafter is referred to as “JP1-315631”). In the case of JP1-315631, the IVC adjustment type variable compression ratio device is exemplified in a two-stroke-cycle Diesel engine. Concretely, when it is determined that the current operating condition of the two-stroke-cycle Diesel engine corresponds to an engine starting period, intake valve closure timing IVC is phase-advanced towards a timing value near bottom dead center (BDC) by means of an electric-motor driven variable valve actuation control device (or a motor-driven-variable valve timing control (VTC) system), thereby increasing an effective compression ratio ε′ and consequently enhancing the self-ignitability during the starting period. In contrast, during engine normal operation, intake valve closure timing IVC is phase-retarded to decrease the effective compression ratio ε′ and consequently to reduce a fuel consumption rate. The rotary-to-linear motion converter (the ball-bearing screw mechanism) of JP1-315631 is comprised of a warm shaft (i.e., a ball bearing shaft with helical grooves) driven by a step motor, an inner slider (i.e., a recirculating ball nut), recirculating balls provided in the helical grooves, and an outer slider axially movable together with the inner slider and rotatable relative to the inner slider. The other type of variable valve operating device has been disclosed in Japanese document “JSAE Journal Vol. 59, No. 2, 2005” published by Society of Automotive Engineers of Japan, Inc. and titled “Gasoline Engine: Recent Trends in Variable Valve Actuation Technologies to Reduce the Emission and Improve the Fuel Economy” and written by two authors Yuuzou Akasaka and Hajime Miura. The Japanese document “JSAE Journal Vol. 59, No. 2, 2005” discloses various types of variable valve operating systems, such as a helical gear piston type two-stepped phase control system, a rotary vane type continuously variable valve timing control (VTC) system, a swing-arm type stepped valve lift and working angle variator, a continuously variable valve event and lift (VEL) control system, and the like. The VTC and VEL control systems are operated by means of respective actuators for example electric motors or electromagnets, each of which is directly driven in response to a control signal (a drive signal) from an electronic control unit (ECU). Alternatively, the VTC and VEL control systems are often operated indirectly by means of a hydraulically-operated device, which is controllable electronically or electromagnetically.